UV astronomy is essential to studies ranging from our own galaxy to the edge of the Universe. The UV range supplies a richness of experimental data which is unmatched by any other domain for the study of hotter objects in the universe because it samples molecular, neutral and atomic gas at temperatures ranging from 10 to 105 K.
The performance of UV detectors has steadily improved over the last decades in many respects, and space surveillance applications have benefited from this evolution. Nevertheless, current solid-state detectors designed for EUV observations (ranging approximately from 2 to 200 nm), are based on Si semiconductors and exhibit a few major drawbacks that are difficult to overcome within silicon technology. Because EUV signal is more than 5 orders of magnitude weaker than the visible counterpart, Si based charge coupled devices (CCDs) must be used in conjunction with filters in order to screen the visible background, which not only attenuate the EUV signal (and hence provide fairly low detectivity efficiency), but also require a sophisticate instrumentation design in order to suppress the visible background. Although cooling reduces the dark current and prevents degradations from ionizing radiations, it is a difficult and expensive solution in space missions. Si-based CCDs also degrade in space due to radiation damage.
Considering the general engineering requirements and constraints for space surveillance applications—reliability, radiation hardness, light weight, and minimal power usage, the next generation of space surveillance systems require orders-of-magnitude performance advances in detectors, detector arrays, and enabling technologies. AlN appears to be an ideal material for the development of EUV detectors, because AlN possesses the widest direct energy bandgap (˜6.2 eV) among all semiconductors and offers the ability for bandgap engineering through the use of alloying and heterostructure design. AlN detectors would help to circumvent many of the limitations imposed by Si technology. The 6.1 eV bandgap permits the visible background to be intrinsically suppressed and the detectors to operate at room temperature, which drastically relive the harsh requirements on optical filters and cooling hardware and greatly simplify the system design. The compact crystal structure of AlN inherently provides radiation hardness.
It was demonstrated by Prof. Hiramatsu's group of Mie University in Japan that AlGaN ternary alloys out perform GaN in terms of photoresponsivity in the EUV and VUV region due to their wider energy band gaps and their result is shown in FIG. 1. For example, at λ<193 nm, the responsivity of Al0.5Ga0.5N detectors is 16 times higher than that of GaN detectors due to the larger bandgap of Al0.5Ga0.5N than GaN. Thus, it is expected that the detection efficiency of AlGaN based detectors in EUV and UVU region increases with an increase of the Al content and is highest for pure AlN.